Scanning electron microscope

ABSTRACT

A scanning electron microscope having a charged particle beam that when in a state being irradiated toward a sample, a voltage is applied to the sample so that the charged particle beam does not reach the sample. The scanning electron microscope also detects information on a potential of a sample using a signal obtained, and a device for automatically adjusting conditions based on the result of measuring.

RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.12/188,870, filed on Aug. 8, 2008, now U.S. Pat. No. 7,989,768, issuedon Aug. 2, 2011, claiming priority of Japanese Patent Application No.2007-207342, filed on Aug. 9, 2007, the entire contents of each of whichare hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The purpose of the present invention is to provide a charged particlebeam device suitable for reducing focus offset, magnificationfluctuation and measurement length error in the charged particle beamdevice caused by charging on a sample.

2. Description of the Related Art

Recently, as semi-conductor devices, particularly, progress, measuringand inspecting technique of a semi-conductor is more and more increasingits importance. A scanning electron microscope represented by a CD-SEM(Critical Dimension-Scanning Electron Microscope) is a device formeasuring the pattern formed on a semi-conductor device by scanning anelectron beam on a sample and detecting electrons such as secondaryelectrons or the like emitted from the sample. In such a device,although the condition of the device is required to be appropriately setto carry out highly accurate measurement and inspection, among recentdevices, there are samples wherein the charge adheres by irradiation ofan electron beam or influence of a semi-conductor process. Insulatorsamples such as resist, insulating film, low-k material and the like, inparticular, are known as the samples to which the charge is liable toadhere.

Following methods are conventionally devised as methods for measurementof a charging potential. In Japanese Unexamined Patent ApplicationPublication (JP-A) No. 7-288096, a method is disclosed wherein anelectron beam is converged on a sample, the electron beam is scanned onthe sample, a “reflecting electron” signal obtained according toirradiation of the beam is detected by a detector, an amount ofvariation of the detected signal in a predetermined time is determined,and either of the pressure around the sample, irradiation amount of theelectron beam, and the acceleration voltage of the electron beam iscontrolled based on the amount of variation obtained (conventionaltechnique 1). Also, a controlling method wherein charging is detectedand feed-back is applied based on it is disclosed in the U.S. Pat. No.6,521,891 (conventional technique 2). According to it, an electron beamis scanned on a sample, a secondary electron and a backscatteringelectron are detected and an image is formed. The image is obtained byvarying accelerating energy of the electron beam, and acceleratingenergy of the electron beam is varied based on the result of analysis ofthe image, thereby compensation of charging of the sample is pursued. Onthe other hand, a method is exhibited in the gazette for theJP-A-1-214769 wherein non-contact measuring of a potential of a sampleis performed. A metal needle having a sharpened tip, a feed-back circuitfor detecting a field emission current or a tunnel current through themetal needle and to apply a voltage to the metal needle so that thecurrent becomes constant, and a circuit for reading out the metal needlevoltage are provided (conventional technique 3).

-   (Patent Document 1) JP-A-7-288096-   (Patent Document 2) U.S. Pat. No. 6,521,891-   (Patent Document 3) JP-A-1-214769

SUMMARY OF THE INVENTION

Although both of the techniques described in the patent documents 1 and2 relate to the technique wherein a charged amount on a sample ismeasured and conditions of a device are adjusted based on themeasurement, because the charged amount is measured by detecting asignal obtained according to irradiation of an electron beam to thesample, charging is forcibly induced by irradiation of the electronbeam, and a problem that measurement of the charged amount prior toirradiation of the electron beam is difficult is involved.

On the other hand, according to the technique described in the patentdocument 3, although measurement of a potential of the sample surface ispossible without inducing charging by an electron beam, there areproblem that it takes time to make the metal needle get close to asample, and problems of change of a potential of the sample by makingthe metal needle get close to the sample and discharging in the casecharge amount is large.

The purpose of the present invention is to provide a device formeasuring a potential of charge of a sample adhering by irradiation of acharged particle beam or by influence of a semi-conductor processwithout irradiation of an electron beam to the sample, and automaticallycompensating for the conditions (magnification, focus, observationcoordinates) of the device which vary according to charge of the sample.

To achieve the purpose described above, the present invention provides adevice for measuring a potential of a sample using a signal obtainedunder a state a voltage is applied to the sample so that a chargedparticle beam does not reach the sample (hereafter such state may bereferred to also as a mirror state) while the charged particle beam isirradiated toward the sample, and automatically compensating theconditions of the device (magnification, focus, observation coordinates)which vary according to charge of the sample.

A specific method will be described below. The means in accordance withthe present invention consists of two steps described below.

1. A Step for Measuring a Potential

Under the mirror state where a primary electron beam is not incident ona sample, optical parameters (an object point ZC for an objective lens,exciting current of the objective lens I_(obj), a potential of thesample V_(s)=V_(r)+ΔV_(s), a potential of a booster V_(b)) are set tooptional values, and a displacement amount (the arrival point of orbit Hon a detector) or a magnification (the arrival point of orbit G on thedetector) is measured, thereby the potential of the sample iscalculated. The detecting method of a displacement amount and amagnification will be exhibited below.

In measuring a displacement amount and a magnification of a mirrorelectron at the detector position, it is preferable to use a detectorwith a plurality of detecting elements spreading two-dimensionally. Thearrival position or distribution of the mirror electron is obtainedbased on the output signal of the plurality of detecting elements,thereby it becomes possible to obtain the deviation from the referencevalue.

Further, if a deviation amount is arranged to be detected using animage, the deviation amount can be detected more easily. While themirror electron is reflected right above the sample and passes through alens system, it is subjected to influence of a passage of the beam and astructure. To obtain the image, the position of the incident beam may bescanned. Thus, the shape of the structure in the path of the beam isformed as an image. By measuring the size of the structure shapetransferred into the image and the sag of an edge, the displacementamount and the magnification can be measured.

As an example of deriving a potential of a sample from the displacementamount and the magnification detected as above, relation between theretarding potential V_(r) and the displacement amount is shown in FIG.2. The curve A in the drawing was obtained as I_(obj) was fixed to anoptional value (I_(obj1)) when the potential by charging ΔV_(s)=0 V.When charging occurs in a sample, a potential of a sample V_(s) can berepresented by a total of a retarding potential V_(r) and a potential bycharging ΔV_(s), therefore the curve of the displacement amount withrespect to the retarding potential shifts by ΔV_(s). Consequently, ifthe retarding potential V_(r) where the displacement amount becomes 0 onthe detector is obtained, the charging potential ΔV_(s) can be derivedby referring to the displacement amount curve A. Further, by measuringthe displacement amounts at two or more kinds of retarding potentialV_(r) and estimating the shifting amount of the curve A, the chargingpotential ΔV_(s) can be estimated.

Here, if the value I_(obj) is set low (high), the focusing potential ofthe sample becomes high (low) and the position Z_(m) (mirror surface)where the irradiated electron is reflected becomes low (high).Therefore, to improve spatial resolution of measurement of a potential,focus can be adjusted with the value I_(obj) being set low and with themirror surface being made close to the sample.

Further, if a potential is measured with the set value of I_(obj)changing, measuring of the potential distribution V_(axis) on the lightaxis (Z) is possible as well.

2. A Step for Automatically Compensating the Conditions of the Device(Focus, Magnification, Observation Coordinates)

If a wafer is not charged, exciting current of the objective lensrequired for focusing is generally represented by a function shown inthe equation (1).I _(obj) =F(V _(o) ,V _(r) ,Z)  (1)where, I_(obj) is the exciting current of the objective lens when thewafer is not charged, F is the function for calculating the excitingcurrent of the objective lens, V_(r) is the retarding potential appliedto the wafer, and Z is the height of the wafer. The function F can bederived by optical simulation or actual measurement. Because thepotential of the wafer which is not charged is generally of the samepotential with the retarding potential applied to the wafer andsatisfies the equation (1), the regular focus control is possible.However, the exciting current of the objective lens required when thewafer itself is charged is as shown in the equation (2), therefore, thefocus current is different in the charged case and the non-charged case.I′ _(obj) =F(V _(o) ,V _(s) ,Z)  (2)V_(s) is a potential of a sample and can be represented as a total of aretarding potential V_(r) applied to a wafer and the charging potentialΔV_(s).

Consequently, even if the height is detected accurately, the focuscannot be adjusted, therefore, the secondary charged particle image isblurred.

Then, if the potential of the sample V_(s) (=V_(r)+ΔV_(s)) is measuredby the method as described in 1. and observation is performed using theexciting current I′_(obj) obtainable by the equation (2), focus controlbecomes possible even when the wafer is charged.

Although the example described above is the method of feedback of theresult of the measurement of the potential to the exciting current,focus control by feedback of the potential of the sample V_(s) obtainedby measurement of the potential to the retarding potential V_(r) is alsopossible.

Besides, in other case of a SEM using what is called a boosting methodwherein a cylindrical electrode applied with a positive potential isdisposed within an objective lens, focus control is possible byadjusting the positive potential V_(b) applied. Furthermore, othergeneral techniques for adjusting the focus of an electron beam areapplicable.

The case the present invention is applied to magnification control of aSEM will be described.

If the potential of a sample varies by charging, the magnification of ascanning electron microscope varies. When a primary electron beamemitted radially from one point of a crossover plane concentrates to onepoint of a sample surface, if an imaginable emitting point of theprimary beam shifts by one unit distance, the imaginable arrival pointonto the sample surface shifts by M_(obj) unit distance. When theconversion coefficient and the coil current of a scanning deflector arerepresented by K and I_(scan) respectively, the distance between twopoints on the sample can be calculated by the following equation.A=K·M _(obj) ·I _(scan)  (3)Also, M_(obj) can be represented by the following equation.M _(obj) =M(V _(o) ,V _(r) ,Z,S _(charge) ,ΔV _(s))  (4)where, S_(charge) is the area of a charged region. With regard tomagnification variation too, if the function M_(obj) has been derivedfrom optical simulation or an experiment, input current I_(scan) of adeflecting coil with A being made constant can be obtained by theequation (3).

As a third application, image drift control and magnification controloccurring in observation utilizing preliminary charging can be cited.

In observing a contact hole with a high aspect ratio, a phenomenon thatthe bottom of the contact hole cannot be seen occurs. Then, a techniqueis disclosed in the JP-A-5-151927 wherein positive charging is createdon the surface of a sample by irradiation of an electron beam at a lowmagnification beforehand, a secondary electron discharged from thebottom of the contact hole is elevated by an elevating electric fieldformed between the bottom of the contact hole and the surface of thesample, thereby observation of the bottom of the hole is performed.However, because of a potential gradient generated in the preliminarycharging, problems such as magnification fluctuation, drift, or the likeoccur. An explanatory drawing of the mechanism of occurrence of drift ofan image is exhibited in FIG. 9.

The charged region (generally, having a size several tens of μm toseveral hundreds of μm on one side) formed by the preliminary chargingis shown at A in FIG. 9. If a positive charge is accumulated evenly overthe region, the potential distribution with the center part beinghighest as shown in the lower portion of FIG. 9 is formed. When observed(one side of the region of observation is several hundreds of nm toseveral μm) after formation of such potential distribution, the primaryelectron beam is subjected to a force from the potential gradient by thepreliminary charging and is forcibly deflected. As a result, an areadifferent from that of the original area desired is forcibly observed.

If the technique in accordance with the present invention is used, thedistribution of the charging potential can be measured with high spatialresolution without irradiation of the primary electron beam onto thesample. Consequently, the present invention can be applied to measuringthe potential gradient of the observation region prior to observation.If observation coordinates is compensated based on the result ofmeasurement of the potential gradient, the desired observation regioncan be observed. Further, as is exhibited in FIG. 10, the influence ofdrift of an image by charging is inhibited and the bottom of the contacthole with a high aspect ratio becomes possible to be observed byperforming the preliminary irradiation again to eliminate the potentialgradient of the observation region. This method is effective not onlyfor observation of the bottom of the contact hole but also for generaltechnique wherein observation is performed after preliminary charging.

According to the constitution described above, observation of the sampleinhibiting the influence of charging by irradiation of an electron beamis possible, because the potential of the sample is detected from theinformation obtained under the state the charged particle beam is notincident on the sample and optical conditions are controlled.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiment(s) of the present invention will be described in detail basedon the following figures, wherein:

FIG. 1 is an explanatory drawing of the outline of a scanning electronmicroscope;

FIG. 2 is an explanatory drawing of a method wherein a potential of asample is derived from a displacement amount;

FIG. 3 is an explanatory drawing of the optical condition suitable formeasuring a potential with high spatial resolution;

FIG. 4 is an explanatory drawing of focus offset occurring when anopening angle increases;

FIG. 5 is an explanatory drawing of an optical condition suitable formeasurement of a potential in high accuracy;

FIG. 6 is a flowchart of the first embodiment in accordance with thepresent invention (focus control);

FIG. 7 is a flowchart of the second embodiment in accordance with thepresent invention (magnification control);

FIG. 8 is a flowchart of the third embodiment in accordance with thepresent invention (opening angle control);

FIG. 9 is a flowchart of the fourth embodiment in accordance with thepresent invention (deflection fulcrum control); and

FIG. 10 is an explanatory drawing of an example inhibiting the potentialgradient of the observation region by performing preliminaryirradiation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The position of a detector appropriate for implementing the presentinvention will be described.

To improve the spatial resolution of measurement of a potential inaccordance with the present invention, two conditions described belowshould be satisfied.

A mirror surface which is a reflecting surface for the primary electronbeam is put near to a sample.

Spreading of the primary electron on the mirror surface is minimized.

FIG. 3 shows an explanatory drawing of an optical condition (opticalcondition A) resulting in the highest spatial resolution in measuring apotential utilizing the present invention. In the drawing, ZC is anobject point of an objective lens where a detector is positioned. If anarrangement such as shown in FIG. 3 is employed, when focused on thedetector, focusing is adjusted on the mirror surface as well.Consequently, in calculating a potential of a sample from the conditionwith which focus offset is minimized on the detector, if the arrangementexhibited in FIG. 3 is employed, the spatial resolution of measurementof a potential can be improved.

Here, a displacement amount reflected to the detector by variation ofthe potential of the sample is proportional to an open angle of theobject point (under the case focus offset by aberration is negligible).Therefore, if the open angle at the object point is made large, thedetection sensitivity of variation of the potential of the sampleimproves. However, under the optical condition as exhibited in FIG. 3,velocity in the lateral direction is forcibly generated on the mirrorsurface. Therefore, as the open angle of the primary beam is larger, thebeam is reflected at a position (A in FIG. 4) which is higher than themirror surface and the focus is offset at the detecting surface.Consequently, even if the open angle is made large and measuringsensitivity of the potential of the sample is improved, the open anglecannot be made large because the focus offset attributable to the openangle as described above occurs.

To solve the problem described above, an optical condition (opticalcondition B) as exhibited in FIG. 5 can be employed. ZC in the drawingis a crossover plane where a detector is positioned. Then, the excitingamount of an objective lens is adjusted so that the inclination of theprimary electron beam on the mirror surface becomes parallel with thelight axis. If the electron beam is irradiated under the condition, anyprimary electron beam having any angle at the object point is incidentperpendicular to the mirror surface, reflected at the potential surfaceof the same potential, and is converged to the same position on thedetector. Therefore, the sensitivity of measuring the potential can beimproved because the open angle of the primary electron beam used formeasurement can be enlarged. However, because the primary electron beamis widened spatially at the mirror surface, spatial resolutiondeteriorates. Accordingly, if spatial resolution of measuring apotential is important, the potential can be measured by the opticalcondition A, and if measuring accuracy for the potential is important,the potential can be measured by the optical condition B. In addition,because the optical condition optimal for measurement of a potentialusing a mirror electron (specifically, crossover position ZC, boostervoltage V_(b), retarding voltage V_(r) open angle of crossover planeα_(c), and deflection fulcrum Z_(p)) and the optical condition optimalfor observation do not coincide, it is preferable to measure switchingthe optical condition used in measurement of the potential and inobservation.

In measuring a potential in accordance with the present invention, ifthe detector is disposed above the deflector, the mirror electron isscanned on the detector by the influence of the deflector. Therefore itis preferable to dispose the detector between the deflector and theobjective lens in measuring a potential in accordance with the presentinvention.

Preferred embodiments in accordance with the present invention will bedescribed below referring to the drawings.

FIG. 1 is an explanatory drawing of the outline of a scanning electronmicroscope. Although the explanation below is made with an example of ascanning electron microscope (SEM) wherein an electron beam is scannedon a sample, the application is by no means limited to it but possiblyto other charged particle beam device as well such as a FIB (Focused IonBeam) device, or the like. However, according to the polarity of thecharge of the beam, it is necessary to vary the polarity of the voltageapplied to the sample. In addition, FIG. 1 explains only one embodimentof a scanned electron microscope, and the present invention can beapplied to the scanned electron microscope with configuration other thanthat of FIG. 1 in a range within the scope thereof.

In a scanning electron microscope explained in FIG. 1, extractionvoltage is applied between the field emission negative electrode 11 andthe extraction electrode 12, and the primary electron beam is extracted.

The primary electron beam 1 thus extracted is accelerated by theacceleration electrode 13, and is subjected to converging by thecondenser lens 14 and scanning deflection by the upper scanningdeflector 21 and the lower scanning deflector 22. The deflectionintensity of the upper scanning deflector 21 and the lower scanningdeflector 22 has been adjusted to allow two-dimensionally scanning onthe sample 23 with the lens center of the objective lens 17 as afulcrum.

The primary electron beam 1 deflected is further subjected toacceleration by rear stage accelerating voltage 19 in the accelerationcylinder 18 disposed in the passage of the objective lens 17. Theprimary electron beam 1 rear stage accelerated is converged by lensaction of the objective lens 17. The cylindrical electrode 20 isgrounded and forms an electric field between the acceleration cylinder18 for accelerating the primary electron beam 1.

An electron such as the secondary electron emitted from the sample orthe backscatter electron is accelerated in the direction opposite theirradiation direction of the primary electron beam 1 by the negativevoltage (hereafter referred also to as retarding voltage) applied to thesample and by the electric field formed in the gap with the accelerationcylinder 18, and is detected by the detector 29.

The electron detected by the detector 29 is synchronized with thescanning signal supplied to the scanning deflector and is displayed onan image display device not shown. Also, the image obtained is stored ina frame memory not shown. Further, the current or the voltage suppliedor applied to each constituting element of the scanning electronmicroscope shown in FIG. 1 may be controlled by a control devicearranged separate from the main body of the scanning electronmicroscope.

First Embodiment

A method for measuring a potential of a sample using an electron beamwill be described below.

A flowchart of the present embodiment is shown in FIG. 6. Also, anoutline of a charging control device is shown in FIG. 8.

In the step S1, judgment is made whether the reference function FR ofthe acquisition condition to be compensated this time has been stored ornot in the reference function record part 102. If there is no referencedata required for the compensation this time in the record part 102, thereference sample or the uncharged sample is made a mirror state in thestep S100 in the loop 1 with the condition stored in the acquisitioncondition record part 103 being set, and the displacement amount or themagnification against V, is detected by a feature amount arithmetic unit101 in the step S120. The reference function FR obtainable by functionfitting using the obtained displacement amount or the magnification isobtained in the step S130, and is stored in the reference functionrecord part 102 in the step S140. When the reference function FR hasbeen obtained in the loop 1 or there already is the reference functionFR in the step 1, the acquisition condition is read out from theacquisition condition record part 103 by the step S100 of the loop 2after charging of the sample, and the mirror state is set. In the stepS110, the displacement amount or the magnification is detected againstV_(r) by a plurality of numbers using the feature amount arithmetic unit101. In the step S130, the potential of the sample V_(s) is derived fromthe feature amount and the number of references FM obtained by thepotential arithmetic unit 104. In the step S150, the compensated valueof the exciting current I_(obj) is calculated based on the potential ofthe sample obtained using the focus current control device 105, and theexciting amount of the objective lens is adjusted. According to thepresent invention, the focus control can be performed by measuring thepotential of the charged sample by the non-contact electron beam andcompensating the exciting current. With this configuration, the focuscontrol in observing an insulated sample can be performed in a shorttime and without variation in the sample condition.

Though the present embodiment is to derive the potential of the sampleusing the relation between the retarding potential V_(r) and thedisplacement amount or the magnification and to perform the focuscontrol by adjusting the exciting current I_(obj), even if the opticalparameters (retarding potential V_(r) and the exciting current I_(obj))shown above are replaced with other optical parameters, similar effectis expectable.

Second Embodiment

A flowchart of the second embodiment is shown in FIG. 7. Also, anoutline of a charging control device is shown in FIG. 8.

In the step S1, judgment is made whether the reference function FR ofthe acquisition condition to be compensated this time has been stored ornot in the reference function record part 102. If there is no referencedata required for the compensation this time in the record part 102, thereference sample or the uncharged sample is made a mirror state in thestep S100 in the loop 1 with the condition stored in the acquisitioncondition record part 103 being set, and the displacement amount or themagnification against V_(r) is detected by a feature amount arithmeticunit 101 in the step S120. The reference function FR obtainable byfunction fitting using the obtained displacement amount or themagnification is obtained in the step S130, and is stored in thereference function record part 102 in the step S140. When the referencefunction FR has been obtained in the loop 1 or there already is thereference function FR in the step 1, the acquisition condition is readout from the acquisition condition record part 103 by the step S100 ofthe loop 2 after charging of the sample, and the mirror state is set. Inthe step S110, the displacement amount or the magnification is detectedagainst V_(r) by a plurality of numbers using the feature amountarithmetic unit 101. In the step S130, the potential of the sample V_(r)is derived from the feature amount and the number of references FMobtained by the potential arithmetic unit 104. In the step S160, thecompensated value of the deflection current I_(scan) is calculated basedon the potential of the sample obtained using the deflection currentcontrol device 105, and the deflection amount is adjusted. According tothe present embodiment, the magnification control can be performed bymeasuring the potential of the charged sample by the non-contactelectron beam and compensating the exciting current.

Though the present embodiment is to derive the potential of the sampleusing the relation between the retarding potential V_(r) and thedisplacement amount or the magnification and to perform themagnification control by adjusting the deflection current I_(scan), evenif the optical parameters (retarding potential V_(r)) shown above arereplaced with other optical parameters, similar effect is expectable.

In addition, feedback to the magnification of the obtained image may beperformed.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

What is claimed is:
 1. A scanning electron microscope comprising: adeflector for scanning an electron beam, an objective lens for focusingthe electron beam, a detector for detecting electrons obtained byscanning a sample with the electron beam and a control device foradjusting a condition of the objective lens, and adjusting a negativevoltage applied to a sample to control energy of an electron beamreaching the sample, wherein the control device controls the objectivelens as an objective point of the objective lens is located at anarrival position of electrons emitted from the sample on the detector,and applies the negative voltage to the sample so as to reflect theelectron beam without reaching the sample by an electric field formed bythe applied negative voltage.
 2. The scanning electron microscope as setforth in claim 1, wherein the detector is disposed between the deflectorfor scanning the electron beam and the objective lens.
 3. The scanningelectron microscope as set forth in claim 1, wherein the detector iscomposed of a plurality of detecting elements to be arrangedtwo-dimensionally.
 4. A scanning electron microscope comprising: adeflector for moving a scanning area of the electron beam, a lens forfocusing the electron beam, a detector for detecting electrons obtainedby scanning a sample with the electron beam and a control device foradjusting the deflector and adjusting a negative voltage applied to asample to control energy of an electron beam reaching the sample,wherein the control device: applies the negative voltage to the sampleso as to reflect the electron beam without reaching the sample by anelectric field formed by the applied negative voltage, detects adeviation between a reference signal and detected signal in a state ofthe electron beam which does not reach the sample, and controls thedeflector based on the detected deviation.
 5. The scanning electronmicroscope as set forth in claim 4, wherein the detector is disposedbetween the deflector for scanning the electron beam and the objectivelens.
 6. The scanning electron microscope as set forth in claim 4,wherein the detector is composed of a plurality of detecting elements tobe arranged two-dimensionally.
 7. A scanning electron microscopecomprising a deflector for scanning an electron beam, a lens forfocusing the electron beam and a detector for detecting electronsobtained by scanning a sample with the electron beam and a controldevice for adjusting a negative voltage applied to a sample to controlenergy of an electron beam reaching the sample, wherein the controldevice: applies the negative voltage to the sample so as to reflect theelectron beam without reaching the sample by an electric field formed bythe applied negative voltage, detects a deviation from a referencesignal of detected signal affected by a structure which exists on atrajectory of the reflected electrons within the scanning electronmicroscope, without the electron beam reaching the sample, and detectsan electrical potential based on detected deviation.
 8. The scanningelectron microscope as set forth in claim 7, wherein the detector isdisposed between the deflector for scanning the electron beam and theobjective lens.
 9. The scanning electron microscope as set forth inclaim 7, wherein the detector is composed of a plurality of detectingelements to be arranged two-dimensionally.
 10. The scanningelectron-microscope as set forth in claim 7, wherein the control devicecontrols the objective lens as changing an open angle of the electronbeam, when the electrical potential is measured.
 11. The scanningelectron microscope as set forth in claim 7, wherein the control devicemakes a deflection fulcrum of the deflector vary synchronizing withvariation in the negative voltage applied to the sample in measuring thepotential of the sample.